Figure 3: The recombinant sTfR binding activities to VP2 and CPV. Two 96-well plates were coated with VP2 (1 μg/mL) (a) and CPV (1 × 10−5 TCID50/mL) (b), respectively. The different amounts of sTfR (5, 2.5, 1.25, 0.63, 0.31, and 0.16 μg/mL) were added for each well (BSA as a negative control). Other two 96-well plates were also coated with VP2 (1 μg/mL) (c) and CPV (1 × 10−5 TCID50/mL) (d) and were added rabbit anti-VP2 polyclonal antibodies at different amount (1, 2, 5, 10 μg/mL) for antibody blocking test. Then the TfR (2.5 μg/mL) was added for all of the wells except mock control well. The mouse anti-His antibody and goat anti-mouse IgG-AP were used for detection of the bound sTfR fused with His-tag. After development with 4-nitrophenyl phosphate substrate, the optical density (OD) for each well was measured using microplate reader at 405 nm. The sTfR binding activity was indicated by OD values.